Pure rat brain tubulin can be cross-linked by ultraviolet irradiation of tubulin-colchicine complexes at the high wavelength maximum of colchicine to form covalent dimers>trimers>tetrameres. With colchicine concentrations about 3x10(-4)M (mole ratio to tubulin 3-12) and irradiation for 5-10 min at 95-109 mW/cm(2), the yield of dimers is 11-17% and of trimers is 4-6% of the total tubulin. The oligomers show polydispersity and anomalously high apparent molecular masses that converge toward expected values in low- density gels. Maximal dimer yields are obtained with MTC and the decreasing photosensitizing potency is MTC>colchicine>colchicide>isocolchicine>thiocolchicine. Single-ring troponoids also promote dimerization. The initial, low-affinity, binding step of colchicine and its analogues is sufficient to photosensitize tubulin dimerization. Stepwise denaturation of pure tubulin with urea shows that the five readily measured function properties differ markedly in the sensitivity to different urea concentration and suggest stepwise unfolding of the molecule.